1. Field of the Invention
The present invention relates to a display control circuit having a superimposing/display capturing function for superimposing another image on an external image and displaying the resultant image.
2. Description of the Related Art
Heretofore, in the case of displaying an external image such as a television broadcast image on a display of an information processing apparatus such as a personal computer and a portable information terminal or a display apparatus of a television broadcast receiver, etc., a superimposing function has been used in order to superimpose a different image on the external image and display the images simultaneously.
FIG. 7 shows an exemplary constitution for carrying out the superimposing function for the television broadcast image in the conventional information processing apparatus having a liquid crystal display (hereinafter, referred to as "LCD").
A tuner section 1 comprises a television broadcast wave receiving antenna 2 and a tuner block 3. The television broadcast wave receiving antenna 2 receives a television broadcast wave transmitted by an NTSC system (National Television System Committee system: employed in Japan, U.S.A., etc.), a PAL system (Phase Alternation by Line system: employed in some countries in Europe) or the like. The tuner block 3 detects a component associated with the image from the television broadcast wave received by the television broadcast wave receiving antenna 2 and then extracts an analog signal. The analog signal converted by the tuner block 3 includes tuner image data and a control signal such as a horizontal synchronizing signal HSYNC, a vertical synchronizing signal VSYNC and a data latch clock DCLK.
The analog signal output from the tuner section 1 is input to an on-screen display 4 (hereinafter, referred to as "OSD"). The OSD 4 stores preset superimpose data such as a channel display in its internal ROM or the like. When the superimposing function is set to ON by software, the OSD 4 superimposes the superimpose data on the input tuner image data in the state of the analog signal and then outputs the resultant image data as external image data. On the other hand, when the superimposing function is set to OFF, the tuner image data is output as the external image data as it is. An A/D converter 5 converts the external image data input from the OSD 4 into a digital signal and then outputs the digital signal to a display control circuit 6. The display control circuit 6 comprises a tuner corresponding type LCD controller 7 and a display dynamic RAM 8 (hereinafter, referred to as "DRAM"). The tuner corresponding type LCD controller 7 includes an external image data latch 9, a timing generator 10 and a display data latch 12.
The digitized external image data is temporarily latched in the external image data latch 9. At the same time, the control signal such as the horizontal synchronizing signal HSYNC, the vertical synchronizing signal VSYNC and the data latch clock DCLK is input to the timing generator 10. In accordance with the input control signal, the timing generator 10 generates a timing signal for storing the external image data latched in the external image data latch 9 in the display DRAM 8. The timing generator 10 also generates a timing signal for outputting the image data to a thin film transistor (hereinafter, referred to as "TFT") liquid crystal panel 15. The display data latch 12 temporarily latches display data stored in the display DRAM 8 therein and then outputs the display data to the TFT liquid crystal panel 15 in accordance with the timing signal generated from the timing generator 10.
FIG. 8 shows an exemplary constitution for carrying out the superimposing function for the television image in a conventional liquid crystal television broadcast receiver. The same elements as the elements in FIG. 7 are denoted by the same reference numerals in FIG. 8, and the description thereof is omitted.
The liquid crystal television broadcast receiver does not need the display DRAM. A tuner corresponding type LCD controller 16 includes a TFT panel controller 17 as well as the external image data latch 9 and the timing generator 10. The external image data temporarily latched in the external image data latch 9 is output directly to the TFT panel controller 17 in accordance with the timing signal generated from the timing generator 10. At the same time, the timing generator 10 outputs a display timing signal to the TFT panel controller 17. The TFT panel controller 17 outputs the external image data to the TFT liquid crystal panel 15 in accordance with the display timing signal.
Disadvantageously, the conventional apparatus for carrying out the superimposing function can superimpose only definite data, for example, the channel display, etc. recorded on the ROM contained in the OSD 4 of FIGS. 7 and 8. Furthermore, when the superimpose data is superimposed on the tuner image data, the superimposition is carried out in the state of the analog signal. Thus, it is impossible to designate a delicate color of the superimpose data, to designate a fine area of the superimpose data and to set other options. Therefore, the superimposed displayed image is prone to blur.
The prior art for partially solving the above-described problem is a liquid crystal television drive circuit disclosed in Japanese Unexamined Patent Publication JP-A 1-289383(1989). This liquid crystal television drive circuit performs the superimposition by the use of a digital code. The image data digitized by the A/D converter is synthesized with digital character data for converting the digital code by the use of a character generator and a parallel/serial converting circuit, whereby the character data and image information are displayed at the same time. Since this liquid crystal television driving apparatus superimposes superimpose data, that is, the character data on the image data in the state of the digital signal, the above-described problem is partially solved. However, since the superimpose data is the digital data for converting the character data in parallel/in series, the contents of the superimpose data are basically limited to character information.
Moreover, when a display screen is captured by the conventional information processing apparatus shown in FIG. 7, since the external image data is once stored in the display DRAM 8, the display screen can be captured into the information processing apparatus without any need for a special operation. However, since the display DRAM 8 performs both capture and read of the image data, the display DRAM 8 is operated at very high speed and thus a current consumed for refreshing the display DRAM 8 is increased. More specifically, the DRAM operated at low speed is desirable for the display DRAM incorporated in the portable information terminal.